1. Field of the Invention
This invention relates to an unsaturated polyester resin composition, more particularly an unsaturated polyester resin composition capable of providing a cured product having a superior surface quality and a sheet-form molding material using the same for producing a fiber-reinforced plastic molded product.
2. Description of the Related Art
Unsaturated polyester resins are accompanied with 5 to 10% volume shrinkage when they are cured. Thus, usually the so-called low shrinking agent-blended unsaturated polyester resins, obtained by blending a thermoplastic resin such as polystyrene, polyvinyl acetate, polymethyl methacrylate, or block copolymers of conjugated dienes as a low shrinking agent with aromatic vinyl compounds, saturated polyesters, etc. have been used. Such low shrinking agent-blended unsaturated polyester resins have a very small shrinkage factor of cured volume, so that the occurrence of cracks, deformation, etc. accompanying the curing shrinkage are minimal.
When the low shrinking agent-blended unsaturated polyester resins represented by sheet-molding compound (SMC) and bulk molding compound (BMC) are used as a molding material, it is possible to improve the dimensional accuracy and the surface quality of molded products; hence the resins have been used not only for parts of housing facilities such as a bath, water tank panel, toilet bowl, etc., but also for parts of automobiles such as a spoiler, air intake, roof, engine hood, deck lid, etc.
The effectiveness of the low shrinking agent varies depending upon the type and blended quantity of the low shrinking agent. As a low shrinking agent for unsaturated polyester resin molding material, polyvinyl acetate, saturated polyester resins, conjugated diene compounds, etc. have been used in the aspect of the surface quality of molded products.
However, when polyvinyl acetate or conjugated diene compounds are used as a low shrinking agent, the viscosity of the resulting unsaturated polyester resin composition increases and, the rate for impregnating fiber reinforcing materials with the resin composition is reduced so that the surface quality of the molded products is inferior. Further, when saturated polyester resins are used as a low shrinking agent, the viscosity of the resulting resin composition is low and the rate for impregnating fiber reinforcing materials with the resin composition is superior, but in the thickened material obtained by reacting the resin composition with an alkaline earth metal oxide or hydroxide, the saturated polyester resins are separated off from the resin composition, so that the low shrinking effect is insufficient and the surface quality of the resulting molded products is notably inferior. As countermeasures for solving such problems, a process of using a saturated polyester resin having a low molecular weight (Japanese patent application laid-open No. Sho 59-122514), a process of reacting an unsaturated polyester resin with a saturated polyester resin at a high temperature (120.degree. to 180.degree. C.) (Japanese patent application laid-open No. Sho 63-43948), etc. have been proposed, but the resulting low shrinking effect is insufficient.